Ignition timing pulse generator for a contactless ignition system of internal combustion engines

ABSTRACT

An ignition timing pulse generator for use in a contactless ignition system of an internal combustion engine which includes a magnetodynamo. The pulse generator includes a pair of auxiliary timing poles secured to main magnet poles of the magnetodynamo and disposed to oppose each other and to thereby define an airgap therebetween. Pickup means are also provided and are mounted on the stator of the magnetodynamo with the pickup means including a pickup core shielded by at least one magnetic shielding member and a first pickup coil wound on the pickup core. A second pickup coil is wound on said magnetic shielding member with the first pickup coil being differentially connected to the second pickup coil to produce an ignition timing pulse substantially without any disturbance from stray fields in response to passage of the airgap between the pair of timing poles when the rotor is rotated.

I United States Patent 1151 Fu'ii 1 Feb. 8 1972 [54] IGNITION TIMING PULSE 3,398,353 8/1968 Noddin ..322/91 GENERATOR FOR A CONTACTLESS 3,405,347 10/1968 Swift ..322/91 3,492,518 1/1970 Wayne ..310/168 COMBUSTION ENGINES FOREIGN PATENTS OR APPLICATIONS [72] In ntor: Minor Fuii riy p n 1,336,035 7/1963 France"; ..3l0/168 [73] Asslgnee: g z fifiz Kaisha Karlya Primary ExaminerJ. D. Miller Assistant Examiner-R. Skudy [22] Filed: Mar. 20, 1970 Attorney-Cushman, Darby & Cushman An ignition timing pulse generator for use in a contactless igni- [30] Forelgn Apphmhon Pnonty Dam tion system of an internal combustion engine which includes a May 5 19 japanw "44/337867 magnetodynamo. The pulse generator includes a pair of aux- May 28, 1969 japanm iliary timing poles secured to main magnet poles of the mag- July 3, 1969 Japan ..44/63293 netodynam and diSPOSed w each Other and m thereby define an airgap therebetween. Pickup means are also 52 115.0. .310/153 310/70 310/168 Pm"ided and are the stat" the 322/91 netodynamo with the pickup means including a pickup core 51 1m. (:1. ..1-102k 21/22 Shielded by least magnetic Shielding member and [58] Field 6: Search ..310/153 168 169 170 67 PickuP Pick? A Pickup is 310/70 72 ,1 ,1 5 wound on said magnetic shielding member with the first pickup coil being differentiallyconnected to the second [56] References Cited pickup coil to produce an ignition timing pulse substantially without any disturbance from stray fields in response to U TED ES PATENTS passage of the airgap between the pair of timing poles when the rotor is rotated. 3,524,438 8/1970 .lanisch ..123/148 E 3,114,851 12/1963 Santi ..3 10/153 9 Claims, 20 Drawing Figures PATENTEDFEB 8:912 3.641.377

SHEET 1 0F 4 F/GI 2 INVENTOR ATTORNEY IGNITION TIMING PULSE GENERATOR FOR A CONTACTLESS IGNITION SYSTEM OF INTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an ignition control system of a variable-speed-type internal combustion engine such as the one used for automobiles, and particularly to an ignition timing pulse generator used in a contactless ignition system provided with a magnetodynamo and a storage capacitor.

2. Description of the Prior Art Generally, a contactless ignition system for an internal combustion engine includes a magnetodynamo for charging a capacitor and an ignition timing pulse generator which produces a pulse signal in synchronism with the rotation of the engine to trigger the discharge of the capacitor, as will be explained in more detail hereinafter.

SUMMARY OF THE INVENTION The main object of the present invention is to provide an ignition timing pulse generator which is incorporated in the magnetodynamo of an internal combustion engine in an ingenious manner and which is simple in structure as well as extremely stable in timing operation and therefore free from adjustments for any speed of the engine.

Another object of the present invention is to provide such a pulse generator provided with a shielded pickup means which is especially immune to spurious magnetic fields.

In order to attain these objects, the ignition pulse generator of the present invention comprises a pair of timing poles having a gap therebetween and secured to a pair of adjacent pole pieces of the rotor of a magnetodynamo, and a magnetically shielded pickup means secured to the stator of said magnetodynamo in opposed relationship with the locus of the rotation of said timing poles.

Features and merits of the invention will be apparent from the following description given in connection with various embodiments of the invention shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an electric circuit of the capacitor chargedischarge-type ignition system using the timing pulse generator according to the present invention;

FIG. 2 is a perspective view of the rotor in the timing pulse generator according to an embodiment of the present invention;

FIG. 3 is a perspective view of the timing core used in the rotor of FIG. 2;

FIG. 4a is a plan view of the stator which is to be combined with the rotor in the timing pulse generator according to an embodiment of the present invention;

FIG. 4b illustrates the arrangement of the timing pickup means in the stator of FIG. 40;

FIGS. 5a, 5b, 5c and 5d are drawings for explaining the operation of the timing pulse generator according to the present invention;

FIGS. 6a, 6b and 6c show another example of the timing pickup means used with the present invention;

FIGS. 7a and 7b illustrate the waveform of the timing pulse produced by the foregoing examples of the pickup means according to the present invention;

FIGS. 80, 8b and 80 show still further examples of the pickup means used with the present invention; and

FIGS. 90, 9b and 9c are graphs illustrating the various waveforms of the signals induced in the pickup means of the timing pulse generator according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, description will first be made of the capacitor charge-discharge-type ignition system of an internal combustion engine to which the ignition timing pulse generator of the present invention is applied. A voltage produced in the armature 32 of a magnetodynamo A is rectified by a rectifier 61 and charges a capacitor 62. When a controlled rectifier 64 is rendered conductive by a signal produced by a timing pulse generator B, the charge stored in the capacitor 62 is discharged through the primary winding of ignition coil means 63 and the controlled rectifier 64 so as to generate a high voltage in the secondary winding of the ignition coil means 63.

FIGS. 2 and 4a show the rotor and stator of the magnetodynamo constituting a power source. This rotor is common to the timing pulse generator according to the present invention. As shown in FIG. 2, the rotor comprises magnetic cores 51 formed of laminated magnetic sheets such as iron sheets fastened together by rivets 52, and magnets 54 interposed between adjacent magnetic cores so as to form a generally cylindrical assembly. The assembly thus formed is covered by alight alloy casting 53 molded thereover. According to the present invention, a timing core 2 as shown in FIG. 3 and which is formed of a magnetic material such as iron is placed on top of the uppermost magnetic core 5 l and fastened thereto by common rivets 52, in the manner as shown in FIG. 2. The assembly is then molded into the light alloy casting 53. Subsequently, a U-shaped portion 2 provided in the timing core 2 is cut away at the inner end thereof when finishing the inner wall of the stator, so that the timing core 2 is divided into two pieces 2a and 2b. These pieces 20 and 2b have their end faces 2a and 2b opposed to each other to provide timing poles. The gap between the timing poles 2a and 2b is maintained at a fixed distance because the two pieces 2a and 2b of the timing core 2 have been integral with each other prior to the molding. Also, since the timing core 2 is of a relatively small sectional area and therefore capable of reaching sufficient magnetic saturation, the decrement in magnetic flux caused to the magnetic core 5] by the timing core 2 is negligibly small.

The stator, as shown in FIG. 4a, comprises a lamp-lighting and battery-charging armature 31 and an ignition power supply armature 32 opposed to each other, and timing pickup means 33 disposed between the two armatures, but substantially circumferentially of the stator assembly. The timing pickup means 33 has an interior construction as shown in FIG. 4b, in which the pickup means comprises pickup cores 34a and 34b formed of a magnetic material such as iron and connected together by a connecting portion 340, a pickup coil 35 wound on the connecting portion 340, and a magnetically shielding casing 36 for magnetically shielding the pickup cores and pickup coil. When assembled together, the stator and the rotor are arranged in such relationship that the pickup cores 34a and 34b of the stator have their end faces opposed to the end faces of the timing poles 2a and 2b of the rotor timing cores 2a and 2b shown in FIG. 2.

Discussion will now be made of the operational principle of the timing pulse generator according to the present invention.

Referring to FIG. 5a, the positional relationship between the magnetic poles of the rotor is typically shown in developed view. As shown, there are power-generating poles N and S on the left-hand and right-hand sides respectively. Thus, the timing cores 2a and 2b have their polarity indicated by S and N respectively. If the magnetic potential difference between the poles N and S is H the magnetic potential between the generating poles N and S and that between the magnetic poles 2a and 2b of the timing cores 2a and 2b are represented by curves a and b respectively, as shown in FIG. 5b. A typical form of the timing pickup means 33 is shown in FIG. 5c. Assuming that the end faces of the cores 34a and 34b of the timing pickup means 33 are respectively opposed to the poles S and N of the rotor timing cores 2a and 2b, there is produced across the pickup cores a magnetic flux provided by the magnetic potential difference I-I between the timing core poles N and S.

A magnetic potential H is applied to the timing pickup cores by the generating poles N and S. Since the ratio of H,

and H is substantially great, the influence of the generating poles N and S is small. The magnetically shielding casing 36 serves to nullify a magnetic potential difference H applied thereto by the generating poles N and S, and this means that the magnetic flux of the timing cores located within the magnetically shielding casing 36 is affected in no way by the generating poles N and S. Also, the gap between the poles N and S of the timing cores is sufficiently smaller than the dimensions of the opening portion of the magnetically shielding casing 36, and therefore the pickup core means is free from the disadvantage that its magnetic flux is reduced by the magnetically shielding casing 36. As a result of the described function provided by the magnetically shielding casing 36, only the magnetic flux provided by the timing cores is allowed to enter the pickup core means. For convenience of description, it is assumed that the rotor is fixed in a position as shown in FIGS. 5a and 5b and that the timing pickup means 33 is displaced rightwardly as shown by the arrow in FIG. 5c,.where.

the internal combustion engine is in its forward rotation. Then, the magnetic flux passing the pickup cores 34a and 34b is varied so that there is produced in the pickup coil 35 a voltage having the same pulse waveform for the opposite polarities as shown in FIG. 51!. If, for example, the magnetic gap between the timing cores is 1 mm. and the diameter of the circular locus followed by the timing cores is 80 mm., then the angle 1 at which the timing core poles pass the pickup cores will be (l/801r) 360=l.43. Thus, the generator as a timing pulse generator can provide a sufficiently large output, even for a low engine speed. Also, the angular variation during the ignition period can be selected within the range of shown in FIG. d. More specifically, the angular variation during the ignition period is 6/4 036, which means great stability.

However, complete magnetic shielding is not desirable either from an economic viewpoint or in respect of the space occupied, and it is practically difficult to achieve. Such a difficulty can be overcome by the pickup means arranged as shown in FIGS. 6a, 6b and 6c. This alternative form of the timing pickup means indicated by 33a comprises a rodlike pickup core 34 formed of a magnetic material such as iron, a pickup coil 35a wound on the pickup core 34, a pickup coil 35b wound on an inner magnetically shielding member 36a in the opposite direction to that in which the pickup coil 35a is wound, and an outer magnetically shielding casing 36b for shielding the pickup coils 35a and 35b as well as the inner magnetically shielding member 36a and constituting a magnetic circuit together with the pickup core 34. When assembled together, the end face of the pickup core 34 and the end face 0 or d of the outer magnetically shielding casing 36b are respectively opposed to the end faces of the timing poles 2a and 2b of the rotor timing cores 2a and 2b, as shown by the imaginary lines in FIG. 6a. The pickup coils 35a and 35b are connected together for differential operation as shown in FIG. 60.

There are provided various magnetic fluxes interlinking the pickup coil 35a, including a main magnetic flux 11 (hereinafter simply referred to as 4),) produced by the magnetic potential difference H, provided by the timing core 2, and a magnetic flux (1) (hereinafter referred to as provided by disturbing magnetic fluxes (shown by arrows 38a, 38b and 38c in FIG. 60) passing from the generating poles laterally through the outer magnetically shielding casing 36b due to incomplete magnetic shielding. and cause voltages Eal and Ea2 to be produced in the pickup coil 35a. If the number of revolutions per minute of the internal combustion engine is N and the structural constant is K these voltages will be represented by the following equations:

Assume that the voltages produced by d) and (1: for a low speed of the internal combustion engine are E and E and that the voltages produced for a high speed of the engine are E and E Then, the relation E, E,,- is often experienced by an internal combustion engine whose speed variation is in a very wide range, thus resulting in erroneous ignition timing. Therefore, if the ignition circuit is set so that ignition can take place at the voltage E in order to ensure a good igniting action even for a low speed of the engine, this voltage E for setting the ignition circuit will be overcome by the voltage E which provides a disturbing voltage when the engine is at a high speed, thus causing a malfunction. In the shown pickup means, the pickup coil 35b is interposed between the. outer and inner magnetically shielding elements and wound on the.

inner one 360, as described above. 'As regards the magnetic fluxes interlinking the pickup coil 35b, the magnetic flux providing a signal equals as in the case of the pickup coil 35a, while the magneticflux (hereinafter simply referred to as (#3) entering laterally through the outer magnetically shielding casing 36b is sufficiently great with respect to (I); in accordance with the difference in cross-sectionalarea between the cores having the pickup coils 35a and 35b wound thereon and with the different magnetic shielding effects to which the pickup coils 35a and 35b are subjected, namely the double shielding effect for the former coil and the single shielding effect for the latter. If the structural constant is K then voltages Ebl and Eb2 produced in the pickup coil 35b by d), and d); are represented as follows:

If the number of turns for the pickup coil 35b is selected so as to provide a relation Ea2=Eb2, that number will be considerably smaller than the number of turns for the pickup coil 35a for the reason set forth above. As a result, the voltage Ebl produced in the pickup coil 35b by (b, is smaller than the voltage Eal similarly produced in the pickup coil 35a, in proportion to the difference between the number of turns of the coil 35b and that of the coil 35a. Therefore, if the pickup coils 35a and 35b are connected together so that the voltages produced therein are differential, and if the voltage produced in the pickup coil 35a is and the voltage produced in the pickup coil 35b is a VEBEbl+Eb2-K-. +K-

then the voltage E appearing at the output terminal of the timing pickup means 33 will be:

Thus, the output voltage is lower by Ebl or K (d, /dN) for Eal, whereas there is provided an extremely stable ignition timing signal which is affected in no way by the magnetic flux from the generating poles.

The use of the pickup means described above provides a timing pulse having a polarity as shown in FlG. 7a. Therefore, if the drive shaft of the generator is rotated in the opposite direction, the variation in the magnetic flux will also be opposite to that during the positive rotation and the output voltage of the timing pickup means 33 will also show the opposite polarity as indicated in FIG. 7!). Thus, the reverse rotation of a two-cycle internal combustion engine can be completely prevented by utilizing only one of the polarities of the voltage produced by the timing pulse generator as the gate control signal for the rectifier 64 provided with control poles shown in FIG. 1.

It will be apparent from the foregoing discussion that the timing pickup means may also be arranged as shown in FIGS. 8a and 8!). A further example of the pickup means indicated by 33b comprises a rodlike pickup core 34, a pickup coil 35a wound on the pickup core 34, a magnetically shielding casing 36a and a pickup coil 35b wound around the magnetically shielding casing 36a in the opposite direction to that in which the pickup coil 35a is wound. The magnetically shielding casing 36a not only magnetically shields the pickup coil 35a but also constitutes a magnetic circuit together with the pickup core 34. The pickup means 33b according to this embodiment operates, of course, substantially in the same way as the pickup means 33a described previously. However voltage Ebl produced in the pickup coil 35b shown in FIG. 8a by d), is substantially zero since a magnetic flux 4:, does not interlink the pickup coil 35b. The pickup coil 35b wound on the magnetically shielding casing 36a may be omitted as shown in FIG. 8c, depending on the construction of the generator or on the condition under which it is put into service.

Using actual numerical values, description will now be made of the performance provided by the present invention in the case where the above-described pickup means 33a is employed. The rotor in use is of the four-pole type having an armature core whose magnetic flux is 21,000 Maxwell, and having a diameter of 80 mm. The timing cores 2a and 2b are l mm. 5 mm. in cross section. The timing pickup means has a pickup coil 35a of 1,000 turns and a pickup coil 35b of 382 turns. When an internal combustion engine was running at a speed of 500 rpm. and 8,000 rpm, measurements were made of the voltages E A s and E H produced in the pickup coil 35a, voltages B and E produced in the pickup coil 35b, and output voltages E and E produced by the pickup means through the differential operation of the pickup coils 35a and 35b. The results are shown in FIGS. 9a, 9b and 90, where the electric angle 9 is represented by the absicissa and the voltages E E and E by the ordinate. In these graphs, the timing signals for 500 rpm. and 8,000 rpm. are shown somewhat deviated from each other with respect to the absicissa for the sake of convenience, although these timing signals actually overlap each other. The letter Trepresents the cycle of one rotation, and E represents the ignition operating voltage. It will be seen that if only the pickup coil 35a is used, the ignition will be effected by the voltage produced by the disturbing magnetic fluxes from the power generating poles for a speed of 8,000 rpm. It will also be appreciated that the timing signal is ideally provided by the differential effect of the pickup coil 35b as shown in FIG. 9c. Further, the difference in number of turns between the pickup coils 35a and 35b means that the core on which the pickup coil 35b is wound has a greater cross-sectional area than the core on which the pickup coil 35a is wound and accordingly the interlinking magnetic flux from the generating poles is greater in the pickup coil 35b than in the pickup coil 35a.

While the foregoing embodiments have been shown and described with respect to the case where the timing pickup means is mounted to the stator internally of the rotor, the timing pickup means may also be mounted to the stator externally of the rotor.

As has been disclosed above, the present invention enables a common rotor to be used with the magnetodynamo and timing pulse generator, thus eliminating the necessity of providing an additional rotor for the timing pulse generator. This leads to the provision of a compact simple-structured and less expensive timing pulse generator. The use of such a common rotor for the timing pulse generator and magnetodynamo further results in a greater peripheral speed of the timing core as well as a sufficient output even for low speeds of the internal combustion engine. Moreover, the employment of the timing pickup means having the above-described compensating coils not only enables the pickup core 34 contained therein to experience a magnetic flux variation when the timing core is closely adjacent thereto, but also enables the pickup core 34 to be free from the influence of the generating magnetic flux to thereby provide a greater peripheral speed of the rotor tim ing core and attain an extremely stable ignition period. A further advantage of the present invention is that there can be produced a timing pulse having a single polarity whereby the reverse rotation of a two-cycle internal combustion engine can be prevented.

What is claimed is:

1. An ignition timing pulse generator for use in a contactless ignition system of an internal combustion engine including a magnetodynamo having a stator and a rotor with main magnet poles, said pulse generator comprising:

a pair of auxiliary timing poles, each secured to a main magnet pole of said magnetodynamo and disposed with respect to each other to define a timing airgap therebetween; and

pickup means mounted on the stator of said magnetodynamo and disposed for response to said timing airgap upon passage thereby, said pickup means comprising a pickup core shielded by at least one. magnetic shielding member, a first pickup coil wound on said pickup core and a second pickup coil wound on said magnetic shielding member, said first pickup coil being differentially connected to said second pickup coil for producing an ignition timing pulse substantially without disturbance from stray magnetic fields in response to the passage of said timing airgap between said pair of timing poles when said rotor is rotated.

2. An ignition timing pulse generator as in claim 1 including outer and inner magnetic shielding members for shielding said pickup core and wherein, said second pickup coil is wound on said inner magnetic shielding member.

3. An ignition timing pulse generator as in claim 1, wherein each of said timing poles extends from a corresponding one of said main magnet poles to form said timing airgap between extended end portions of said timing poles, and said pickup means is disposed opposite to one position of said extended end portions.

4. An ignition timing pulse generator as in claim 3, wherein said timing airgap between said timing poles is relatively small so that said pickup means is substantially responsive only to said timing poles.

5. A magnetodynamo including a contactless ignition timing pulse generator for use with an internal combustion engine, said magnetodynamo comprising;

a stator having main output windings,

a rotor having at least one pair of main magnet pole pieces for producing a main electrical output on said main output windings in response to rotation of the rotor,

a pair of auxiliary timing pole pieces, each timing pole piece being secured to a corresponding main magnet pole piece being secured to a corresponding main magnet pole piece and extending therefrom to define a magnetic timing airgap therebetween, said timing airgap being relatively small with respect to the distance between said main pole pieces, and

pickup means disposed on said stator for producing a contactless ignition timing pulse in response to passage of said magnetic timing airgap, said pickup means comprismg;

a central magnetic pickup core,

at least one magnetic shielding member disposed about said central pickup core for magnetically shielding the pickup core, and

a first pickup coil wound on said central pickup core for producing an electrical voltage in response to the passage of said timing airgap past an end of said pickup core.

6. A magnetodynamo as in claim further comprising:

a second pickup coil wound on said shielding member,

said first pickup coil being differentially connected to said second pickup coil whereby electrical signals in said first pickup coil caused by stray magnetic fields are substantially offset by electrical signals in said second pickup coil caused by said stray magnetic fields.

7. A magnetodynamo as in claim 6 wherein said first and second pickup coils have predetermined numbers of turns and physical dimensions for insuring that the voltage produced by stray magnetic fields in said first pickup coil is substantially equal to the corresponding voltage produced by stray magnetic fields in said second pickup coil whereby the net output voltage of said first and second pickup coils, when differentially connected, is substantially independent of said stray magnetic fields.

8. A magnetodynamo as in claim 6 wherein said shielding member comprises an inner shielding member and further including:

a further outer magnetic shielding member disposed about said inner shielding member.

9. A magnetodynamo as in claim 6 wherein said auxiliary timing pole pieces are of sufficiently small cross-sectional area to prevent undue magnetic shunting of said main magnet poles to which said timing pole pieces are attached. 

1. An ignition timing pulse generator for use in a contactless ignition system of an internal combustion engine including a magnetodynamo having a stator and a rotor with main magnet poles, said pulse generator comprising: a pair of auxiliary timing poles, each secured to a main magnet pole of said magnetodynamo and disposed with respect to each other to define a timing airgap therebetween; and pickup means mounted on the stator of said magnetodynamo and disposed for response to said timing airgap upon passage thereby, said pickup means comprising a pickup core shielded by at least one magnetic shielding member, a first pickup coil wound on said pickup core and a second pickup coil wound on said magnetic shielding member, said first pickup coil being differentially connected to saId second pickup coil for producing an ignition timing pulse substantially without disturbance from stray magnetic fields in response to the passage of said timing airgap between said pair of timing poles when said rotor is rotated.
 2. An ignition timing pulse generator as in claim 1 including outer and inner magnetic shielding members for shielding said pickup core and wherein, said second pickup coil is wound on said inner magnetic shielding member.
 3. An ignition timing pulse generator as in claim 1, wherein each of said timing poles extends from a corresponding one of said main magnet poles to form said timing airgap between extended end portions of said timing poles, and said pickup means is disposed opposite to one position of said extended end portions.
 4. An ignition timing pulse generator as in claim 3, wherein said timing airgap between said timing poles is relatively small so that said pickup means is substantially responsive only to said timing poles.
 5. A magnetodynamo including a contactless ignition timing pulse generator for use with an internal combustion engine, said magnetodynamo comprising; a stator having main output windings, a rotor having at least one pair of main magnet pole pieces for producing a main electrical output on said main output windings in response to rotation of the rotor, a pair of auxiliary timing pole pieces, each timing pole piece being secured to a corresponding main magnet pole piece and extending therefrom to define a magnetic timing airgap therebetween, said timing airgap being relatively small with respect to the distance between said main pole pieces, and pickup means disposed on said stator for producing a contactless ignition timing pulse in response to passage of said magnetic timing airgap, said pickup means comprising; a central magnetic pickup core, at least one magnetic shielding member disposed about said central pickup core for magnetically shielding the pickup core, and a first pickup coil wound on said central pickup core for producing an electrical voltage in response to the passage of said timing airgap past an end of said pickup core.
 6. A magnetodynamo as in claim 5 further comprising: a second pickup coil wound on said shielding member, said first pickup coil being differentially connected to said second pickup coil whereby electrical signals in said first pickup coil caused by stray magnetic fields are substantially offset by electrical signals in said second pickup coil caused by said stray magnetic fields.
 7. A magnetodynamo as in claim 6 wherein said first and second pickup coils have predetermined numbers of turns and physical dimensions for insuring that the voltage produced by stray magnetic fields in said first pickup coil is substantially equal to the corresponding voltage produced by stray magnetic fields in said second pickup coil whereby the net output voltage of said first and second pickup coils, when differentially connected, is substantially independent of said stray magnetic fields.
 8. A magnetodynamo as in claim 6 wherein said shielding member comprises an inner shielding member and further including: a further outer magnetic shielding member disposed about said inner shielding member.
 9. A magnetodynamo as in claim 6 wherein said auxiliary timing pole pieces are of sufficiently small cross-sectional area to prevent undue magnetic shunting of said main magnet poles to which said timing pole pieces are attached. 